A common concern in the field of LED lighting has been how to keep total harmonic distortion (THD) below 10%. The power supply can be used not only as a non-linear load, but also as a distortion waveform containing harmonics. These harmonics may interfere with the operation of other electronic systems. Therefore, measuring the overall impact of these harmonics is very important. Total harmonic distortion provides us with information about the harmonic content of the fundamental component of the signal wrt. A higher THD means more distortion at the input power supply or lower power quality.
Therefore, I had to test a design method using a 15 W spotlight (insulation) design with a TPS92314 device for 7 series LED configurations that provide 3.1V forward voltage and 0.7A from a 150 ~ 265V AC input. Rated current. Follow the instructions below to achieve a 8.7% THD at an AC input voltage of 240V.
Before proceeding with the actual implementation, please consult this application note for the two important equations required to complete the test.
In this example, k is equal to 1.68, and we can plot the relationship between THD and “m†when k = 1.68 by the above equation.
From the figure below we can see that when k increases (in m
Therefore, looking back at the definitions of "m" and "k", we will find that increasing the turns ratio (n = Np/Ns) and converter delay time can reduce THD. In addition to these two parameters, the EMI filter design can also play a very important role in improving THD. Three design considerations to reduce total harmonic distortion include:
Increasing the transformer turns ratio (n = Np/Ns) increases the reflected voltage. This will increase the cost and voltage stress of the switching FET. In this particular case, we adjust the turns ratio to approximately 10 to maintain the reflected voltage at approximately 174V. The FET rating must be higher than the overshoot voltage, (LED maximum voltage + output diode drop) × turns ratio plus the sum of the peak AC input voltages. The result is approximately 640V [= 50 V + (20 + 0.5) * 10 + 1.414 * 265]. I am using a 700V rated FET and a low drain source capacitor of about 16pF.
Increasing the converter delay time reduces THD. I changed the resistor from the calculated 5.6k to 6.2k. The delay time depends on the primary winding inductance of the transformer and the drain-source capacitance of the FET. The resulting delay time is approximately 280 ns.
Add an EMI filter to the input. In this example, an 80mH common-mode coil with a 275V AC, 68nF capacitor is added to the input, and a π filter containing a 1mH drum inductor and two 400V, 33nF capacitors is added after the bridge. This helps us achieve a 2.15 kHz differential filter corner frequency. With the help of the line impedance stabilization network and the spectrum analyzer, I calculated these values ​​over multiple iterations after looking at the conducted EMI curve. In the initial absence of any line filter, the peak is about 85dBuV at 100 kHz (converter switching frequency). This spectrum has exceeded the limits of the CISPR 15 Class B standard and is not limited to the limit until the frequency is 1 MHz. Therefore, an EMI filter has to be used. I gradually increase the value of the common mode coil and observe its effect on the THD performance (increasing the capacitor to a certain level will reduce the PF performance). Finally, the value is around 80mH and 68nF, and the cutoff frequency is 2.15 kHz, with attenuation exceeding 30dB, reducing the peak at 100 kHz to 55.78dBuV. In this way, the spectrum has not only declined, but it has also enabled the lights to meet the CISPR 15 standard (both quasi-peak and average limits). After this change, THD improved to approximately 9-10%. The leakage inductance associated with the common mode coil helped me implement a differential filter.
By making the above changes, I was able to achieve 8.5% THD and 0.98 PF at an input voltage of 240V with an output voltage of 21.8V. Using six LEDs at the output (18.8V output) in the same design, we achieved 9% THD at 240V. An 80mH EMI filter is implemented with the EE1685 core (turns 180). The primary transformer has a primary inductance of 2mH and a peak main current of approximately 0.5A.
The LED driver used in this test is the TPS92314, a primary-side control offline LED driver designed for low-cost lighting applications (small external components). It features a constant on-time architecture that enables natural power factor correction without the need for complex compensation techniques. In addition, the resonant valley switch reduces EMI and increases system efficiency. Other excellent features include cycle-by-cycle primary side current limit, VCC overvoltage protection and undervoltage lockout, output LED overvoltage protection, and controller shutdown.
The complete schematic based on TI TPS92314 is as follows.
Right Angle DIP Centronic Connector
Right Angle DIP Centronic Connector.
Current Rating:5A
Dielectric Withstanding Voltage:1000V for one minute
Insulation Resistance:1000MΩ Min.(at 500V DC)
Contact Resistance:35mΩ Max.
Temperature:-55°C to +105°C
Right Angle DIP Centronic Connector
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